Travis: make gtk versions explicit
Travis: make gtk versions explicit

EPashkin/rust-gnome-gtk,Susurrus/gtk,gkoz/gtk-rs-gtk,EPashkin/rust-gnome-gtk,rust-gnome/gtk,daa84/gtk,gkoz/rust-gnome-gtk,gkoz/rust-gnome-gtk,Susurrus/gtk,rust-gnome/gtk,daa84/gtk,gkoz/gtk-rs-gtk